I don't know about anyone else, but when I'm rising in the trot the movement of the forward shoulder doesn't correspond exactly with the horse pushing me up. I'm trying to remember which comes first now, but I have a habit of misreading the shoulder so that I'm actually sitting as it's coming forwards. I think this is right (I'll check tonight!), but the horse actually nudges me up just about as the shoulder moves furthest back, I.e. At the very end of the outside fore/inside hind being on the ground.
The main problem I have is reacting too late: I start to rise when I realise the shoulder is coming forwards, which means that as the horse puts me into the right rhythm (rising should never be an effort, always be a natural function of the way the horse pushes you up), I'm on the wrong diagonal. I find the easiest way to avoid this is to concentrate on sitting, rather than rising: it's somehow easier to sit as the shoulder goes back than to rise as I think it's going forwards.
If you believe everything you read, better not read.